A radioiodinated 4-thia fatty acid analog has been previously reported (Gildehaus et al, J Nucl Med 38:124P, 1997, abstract). .sup.18 F-labeled fatty acids are known generally from U.S. Pat. No. 4,323,547 (incorporated hereinto by reference) as useful in PET studies of myocardial metabolism. More recently, .sup.18 F-labeled 6-thia fatty acid (14F6THA) has been synthesized and evaluated (DeGrado, J Lab Compd Radiopharm 24:989-995, 1991; DeGrado et al, J Nucl Med 32:1888-1896, 1991, each incorporated hereinto fully by reference). 4-thia and 6-thia fatty acid analogs track total beta-oxidation of palmitate in the liver. Beta-oxidation of long-chain fatty acids may occur in either mitochondria or peroxisomes. Investigation in isolated rat livers using specific inhibition of carnitine palmitoyl-transferase has suggested that .sup.18 F-labeled 4-thia fatty acids are trapped in the liver tissue in both organelles. A more specific probe of mitochondrial metabolic function is needed.